The present invention relates generally to module construction for assemblies having several-to-many elements, and more specifically to systems and methods for use of selectively curable low viscosity/low surface tension adhesives for assembly construction, most particularly for battery module assembly construction.
It is common to produce module assemblies having elements secured together into an integrated monolithic structure. One method for securing the elements together uses an adhesive that bonds the elements to one or more fixtures. These fixtures typically include many openings that may be used to provide access to the elements, the inside of the assembly, or for other use. To inhibit adhesive from escaping from these openings during manufacture, conventional solutions use adhesives having great enough viscosity/surface tension to inhibit adhesive from exiting the apertures.
In many applications such an adhesive provides an acceptable solution. In other applications, an adhesive like this is problematic. One problem is that the viscosity/surface tension requires a fairly large hydrostatic head in order to direct a suitable quantity of adhesive into small margin bonding areas to adequately wet the bonding surfaces and provide sufficient bond strength. This hydrostatic head represents a large quantity of adhesive that is “wasted” in the sense that it does not contribute to the security of the bonding between the elements and the fixture.
It is not only the case that this adhesive is expensive and therefore any wasted adhesive adds to the ultimate cost. In some applications, like electric vehicles, a further drawback is that excessive adhesive adds “unnecessary” mass to the final assembly while also being unnecessarily expensive. As the number of elements in the assembly increases, and as a packing density of the elements increases, the costs in terms of expense and mass become quite significant because of the multiplicative accumulation of individual excess across all the elements in all of the modules. Any savings in reducing the quantity of adhesive per element/module is extremely effective in these cases as it reduces both expense and mass.
Adhesives have an associated curing profile that further influences the use and suitability of adhesives in module assemblies. There are two broad categories of adhesives—one-part adhesives and two-part adhesives. Two-part adhesives are adhesives that include a base and a hardener. In contrast, a one-part adhesive includes the functionality of both the base and the hardener, but the activation or release of the hardener depends upon some externality for curing (e.g., temperature, ultraviolet radiation, water vapor in the environment, and the like). Epoxy is an example of a two-part adhesive having a resin and a hardener, with the hardener accelerating a polymerization (i.e., curing) of the adhesive, the specifics of the curing can be influenced by temperature and choice of resin(s) and hardener(s). Whether one-part or two-part, each adhesive has a curing modality that produces a rigid and strong bond in response to one or more curing agents. A cure time, particularly a minimum cure time to reach sufficient mechanical integrity for further processing, is an additional cost of use of adhesives. The cure time is sometimes related to the total quantity of adhesive used, using less adhesive can sometimes improve cure time cost.
Further, in order for each battery module to remain mechanically robust in harsh environments such as within an automobile, a structural connection (physical and electrical) between each battery cell and module fixture should be stiff and robust. Many commodity cells do not include mechanical features that easily allow for such connections. For those that do, it is often difficult to make a stiff and robust mechanical connection while maintaining electrical connectivity requirements. Adhesives are able to establish high shear strength between smooth cell surfaces and fixtures of the module system while also maintaining any desired electrical isolation among the cells. However as noted above, adhesives can be expensive when used in this context and often lead to addition of an undesirable amount of mass to each battery module of a multiple-module battery pack. There is an additional cost to be considered, particularly as the number and density of elements in a module increases and still further as the number of these modules increases. This additional cost relates directly to long cure times often required of adhesives, and these long cure times increase process cost.
As noted above, in a closely packed battery module, the quantity of adhesive applied depends in part on the hydrostatic head required to drive wetting of the adhesive to the required bonding surfaces. Some methodologies apply 1-2 mL of adhesive (viscosity approximately 7000 cps) to bond each 18650 battery cell into its own shallow plastic counterbore provided in a fixture. This quantity of adhesive is considered a requirement to achieve coverage of all bonding areas. However, the actual quantity of adhesive required to fill each bonding area alone is approximately 0.050-0.100 mL. Consequently, when considering what is needed for bonding the battery cell to the module fixture alone, a large amount of the adhesive is wasted.
The standard adhesives used in conventional solutions that are outside the bonding areas serves little purpose once the adhesive has sufficiently wetted the bonding surfaces, and this “excess” adhesive has virtually no purpose in the finished product. Reducing a dependency on the hydrostatic head to wet the bonding surfaces in modularized assemblies has a potential to produce significant cost savings by eliminating the “wasted” adhesive.
What is needed is a method and apparatus for decreasing costs (expense, mass, and/or cure time) associated with use of adhesives when assembling modularized components.